(Non)Thermal Aspects of Charmonium Production and a New Look at J/ψ Suppression

To investigate a recent proposal that J/ψ production in ultra-relativistic nuclear collisions is of thermal origin we have reanalyzed the data from the NA38/50 collaboration within a thermal model including charm. Comparison of the calculated with measured yields demonstrates the non-thermal origin of hidden charm production at SPS energy. However, the ratio ψ ′ /(J/ψ) exhibits, in central nucleus-nucleus collisions, thermal features which lead us to a new interpretation of open charm and charmonium production at SPS energy. Implications for RHIC and LHC energy measurements will be discussed. The suppression of J/ψ mesons (compared to what is expected from hard scattering models) was early on predicted [1] to be a signature for color deconfinement. Data for S-induced collisions exhibited a significant suppression but systematic studies soon revealed that such suppression exists already in p-nucleus collisions and is due to the absorption in (normal) nuclear matter of a pre-resonant state consisting, e.g., of a color singlet c¯cg state that is formed on the way towards J/ψ production. The situation has been summarized in [2–4].